Conventionally, as a laminate for a printed wiring board, an FR-4 type laminate obtained by curing an epoxy resin with dicyandiamide has been widely used. However, this method had a limitation in response to the demand for high heat resistance. Also, a cyanate ester resin is known as a resin for a printed wiring board with excellent heat resistance, and a prepreg using a resin composition of a bisphenol A type cyanate ester resin and other thermosetting resins or thermoplastic resins has been widely used for a laminate for a semiconductor plastic package in recent years.
Moreover, high integration, high function and high density implementation of semiconductors widely used for electronic devices, communication instruments, personal computers and the like have recently been increasingly accelerated, and the form of a semiconductor plastic package is getting to be diverse as seen in development from QFP to the area implementation type such as BGA and CSP, and additionally appearance of the high function type such as MCP and SIP. Therefore, the demand for high reliability of a laminate for a semiconductor plastic package has increased more than ever.
A semiconductor element has a coefficient of thermal expansion of 3 to 6 ppm/° C., which is smaller than the coefficient of thermal expansion of a typical printed wiring board for a semiconductor plastic package. Therefore, when a thermal shock is added to a semiconductor plastic package, warpage is generated in a semiconductor plastic package due to the difference in the coefficient of thermal expansion between a semiconductor element and a printed wiring board for a semiconductor plastic package, and poor connection may occur between the semiconductor element and the printed wiring board for the semiconductor plastic package, and between the semiconductor plastic package and the printed wiring board implemented therewith. Thus, in order to reduce the warpage to ensure connection reliability, development of a printed wiring board with a small coefficient of thermal expansion in the surface direction is required.
A method for reducing the coefficient of thermal expansion in the surface direction of a laminate for a printed wiring board includes a method of filling a resin composition with an inorganic filler. However, a large filling amount of an inorganic filler has problems of making a resulting resin composition fragile, deteriorating drilling quality for forming a through hole required for interlayer connection of a printed wiring board, and also quickening wear of a drill bit used for machining to significantly decrease productivity of machining. As another method for reducing a coefficient of thermal expansion in the surface direction, it is known to blend an organic filler with rubber elasticity in a varnish containing an epoxy resin (Patent Documents 1-5), and when this varnish is used, a bromine-based flame retardant has sometimes been used for flame retardance of a laminate.
As described above, conventionally, in order to impart flame retardance to a laminate, a formulation using a bromine-based flame retardant in combination has been used. However, in concert with the recently heightened environmental issues, a resin composition using no halogen-based compound has been demanded, and in response to this demand, the use of a phosphorous compound as a substitute for a halogen-based flame retardant has been considered. However, since a phosphorous compound is liable to generate a toxic compound such as phosphine upon combustion, development of a laminate having flame retardance without using a halogen-based compound or a phosphorous compound has been desired.
With the aim of developing a laminate having flame retardance without using a halogen-based compound or a phosphorous compound, a cyanate ester resin has been considered, and a novolac type cyanate ester resin (Patent Document 6) and a naphthol aralkyl type cyanate ester resin are known. However, a phenol novolac type cyanate ester resin, in which a cyanate group equivalent is small, and many unreacted cyanate groups tend to remain upon curing due to its rigid framework structure, is not satisfying in properties such as adhesion to a metal foil, water resistance and heat resistance under moisture absorption. On the other hand, by utilizing a characteristic that the resin framework is a rigid structure, a resin composition comprising a naphthol aralkyl type cyanate ester resin can maintain heat resistance, as well as reducing reaction inhibition factors to enhance curing property, and has a characteristic of being excellent in water resistance and heat resistance under moisture absorption (Patent Document 7).